math/big: better documentation for Rat.Float64
math/big: better documentation for Rat.Float64

R=adonovan, r
CC=golang-dev
https://codereview.appspot.com/11961043

pombredanne/go-deleteme,pombredanne/go-deleteme,apparentlymart/golang,apparentlymart/golang,tv42/old-go,apparentlymart/golang,pombredanne/go-deleteme,apparentlymart/golang,tv42/old-go,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,apparentlymart/golang,pombredanne/go-deleteme,apparentlymart/golang,pombredanne/go-deleteme,apparentlymart/golang,tv42/old-go